A or an can be precede only singular count nouns; they mean one. They can be used in a general statement or to introduce a subject which has not been previously mentioned.
A baseball is round. (general—means all baseballs)
I saw a boy in the street. (We don’t know which boy)
An is used before words that begin with a vowel sound. A is used before words that begin with a consonant sound.
A book an apple
Some words can be confusing because the spelling does not indicate the pronunciation.
A house (begins with a consonant sound)
An hour (begins with a vowel sound)
A university (begins with a consonant sound)
An umbrella (begins with a vowel sound)
The following words begin with a consonant sound and thus must always be precede by a.
European; eulogy; euphemism; eucalyptus;
house; home; heavy; half;
uniform; university; universal; union
The following words begin with a vowel sound and thus must always be precede by an.
hour; heir; herbal; honor;
uncle; umbrella; unnatural; understanding
the initial sound of the word that immediately follows the indefinite article will determine whether it should be a or an.
an umbrella; a white umbrella
an hour; a white hour